“It’s not the heat, it’s the humidity.”  Ever heard that phrase?  I hear it all the time in the summer, and have even said it several times on TV.  It refers to the fact that high humidity makes it feel hotter than the actual air temperature.

Have you ever wondered why?

The answer – evaporation.  Our body produces sweat to help keep us cool, but that only works if the sweat evaporates, because evaporation is a cooling process.  So when the relative humidity of the air is high, meaning the air has a high moisture content, the sweat evaporation process slows down.  The result?  It feels hotter to you.  The opposite occurs if the air is very dry.  Even on a 100 degree day, it can feel a little cooler to the body if the air is dry because sweat evaporates quickly.

If you’ve been watching “Weather on the 1s” lately, you’ve probably heard us use the term “heat index” or “feels like” temperatures to describe how hot it feels.  Want to calculate the heat index on your own?  Be my guest.  Here’s the equation, where T is air temperature (F), and R is relative humidity (%):

Heat Index = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783 x 10-3T2 - 5.481717 x 10-2R2 + 1.22874 x 10-3T2R + 8.5282 x 10-4TR2 - 1.99 x 10-6T2R2

Or, you could simply reference this chart:

If you’re wondering why the humidity is so persistent in our region this time of year.  It’s because our prevailing wind is from the S/SE, meaning Gulf moisture is constantly being pushed our way.  In our region, the heat index is considered to be dangerous when it exceeds 105.  On those days there will usually be a heat advisory issued by the National Weather Service.  In more extreme heat and humidity, they would issue an excessive heat warning.